Pixelated layer on image in ios

I need to add pixelated rectangular layer on UIImage which can be undo. Just like this..

I used this code but its not doing the same thing as i need

    CALayer *maskLayer = [CALayer layer];
    CALayer *mosaicLayer = [CALayer layer];

    // Mask image ends with 0.15 opacity on both sides. Set the background color of the layer
    // to the same value so the layer can extend the mask image.
    mosaicLayer.contents = (id)[img CGImage];
    mosaicLayer.frame = CGRectMake(0,0, img.size.width, img.size.height);
    UIImage *maskImg = [UIImage imageWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"mask" ofType:@"png"]];
    maskLayer.contents = (id)[maskImg CGImage];
    maskLayer.frame = CGRectMake(100,150, maskImg.size.width, maskImg.size.height);

    mosaicLayer.mask = maskLayer;

    [imageView.layer addSublayer:mosaicLayer];

    UIGraphicsBeginImageContext(imageView.layer.bounds.size);
    [imageView.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *saver = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

is there any built-in filter by apple for iOS? Please guide me Thanks